Despite their relatively recent introduction, ultrafast (e.g., femtosecond-range pulse-width) high-intensity lasers are rapidly becoming important tools for both research and industry. Such laser systems are often referred to as “Ultrafast Kilohertz” lasers (or kHz lasers), referring to their pulse repetition rate in the kilohertz range, or as “femtosecond” (fs) lasers, referring to their very short pulse width, anywhere from tens to hundreds of femtoseconds (1 fs=10−15 seconds, or one one-thousandth of a picosecond). Only a little over decade after their invention, there are thousands of such lasers operating worldwide in research and industrial applications.
New applications for kHz femtosecond lasers are constantly demanding ever-higher levels of laser energy/peak power from the kHz laser. In response, manufacturers of such lasers continually improve their products to meet the market demand. Modern state-of-the-art commercial kHz femtosecond Ti:Sapphire lasers are capable of producing pulses of 2.5 mJ per pulse at 1 kHz using a two-stage pulse amplification system (1 mJ 1 millijoule or 10−3 Joules). Systems that employ single-stage amplification systems are typically limited to pulse output levels of 1.0 mJ to 1.5 mJ per pulse.
Peak output power is defined as the ratio of pulse energy to pulse duration (pulse width). Accordingly, short pulse width (pulse duration) is an important, even critical, factor in achieving high peak output power. Many “high-power” lasers produce high pulse energy, but at a “wider” (relatively speaking) pulse width on the order of 100 fs, thereby reducing their peak output power considerably compared to peak output power levels that would be achieved if the same pulse energy was delivered in a shorter pulse (e.g., 20–30 fs—the pulse width produced by many “lower average power”, kHz femtosecond lasers).
The first high-power femtosecond laser was developed in 1991 by J. Squier et al (J. Squier et. al, “100-fs pulse generation and amplification in Ti:Al2O3,” Optics Letters, 16, 324 (1991)), using a single stage regenerative amplifier. It was capable of producing pulses of 1.1 mJ at a pulse width of 105 fs. Its peak output power is 1010 W, and exhibited amplifier efficiency of 20%.
Although regenerative amplifiers facilitate high output pulse energy and high overall efficiency, they do not produce “ultrashort” pulses easily (i.e., pulses of significantly less than 100 fs in duration) because regenerative amplification introduces considerable high-order dispersion. Clark-MXR, Inc. of Dexter, Mich., a major provider of kHz femtosecond lasers, employs a single-stage regenerative amplifier in their current CPA2010 laser system, achieving output levels of 1 mJ and 150 fs per pulse at a repetition rate of 1 kHz.
In 1995, M. Lenzner et al developed a 1 kHz laser using a single-stage multi-pass amplifier that produced 0.1 mJ pulse energy and 18 fs pulse duration (M. Lenzner et al., “Sub-20-fs, kilohertz-repetition-rate Ti:sapphire amplifier”, Optics Letters, 20, 1397 (1995)). The advantage of a multi-pass amplifier over a regenerative amplifier is that it is relatively easy to produce extremely short pulses because the multi-pass amplifier introduces considerably less material dispersion than the regenerative amplifier. However, it is difficult to achieve high efficiency due to the limited number of passes through the gain medium. A similar system is employed by Femtolasers GmbH of Vienna, Austria to deliver 30 fs, 1 mJ pulses at 1 kHz.
A two-stage system employing a regenerative amplifier followed by a multi-pass amplifier capable of 94 fs, 5 mJ pulses was reported by Fu et al. in 1996 (Fu et al., “High average-power kilohertz-repetition-rate sub-100 fs Ti:sapphire system”, Optics Letters, 712 (1997)). Positive Light, Inc. of Los Gatos, Calif. produces a laser system using a similar design to produce 2.5 mJ, 30 fs pulses.
Researchers have built lasers using two or more stages of amplifiers to produce even higher peak power. The higher peak power produced to date by a kHz laser is 1.1×1012 W (V. Bagnond and F. Salin, “1.1 Terawatt, kilohertz femtosecond laser”, presented at “Conference on Lasers and Electro-optics”, 1999, Baltimore Md.).
High power, kHz femtosecond lasers typically achieve high-power output pulses by amplifying a “small” laser seed pulse from a laser oscillator (a low-power seed pulse, typically on the order of nanoJoules (nJ)) to pulse energy on the order of mJ. This amplification is usually accomplished by means a population inversion in a gain medium produced by directing a pump laser at the gain medium. The seed pulse is passed through inverted gain medium to gain energy, thereby achieving pulse amplification.
To amplify a signal from the nJ level to the mJ level requires total power amplification of approximately 1,000,000:1. It is not possible to achieve such great amplification in a single pass through a gain medium. Even under the best of circumstances, single-pass amplification is on the order of 10:1 and is typically lower. High levels of pulse amplification are achieved by directing the seed pulse through the inverted gain medium in multiple passes, each time gaining in energy. For very high power systems, a second stage amplifier is typically employed, with the first stage providing high gain pre-amplification and the second stage providing power amplification.
When compared to lasers of comparable power output and efficiency, lasers using a single-stage architecture are less expensive, easier to manufacture and easier to operate than those employing two or more stages of amplification. Lasers that employ a single-stage amplifier use considerably fewer parts. The simplicity of single-stage amplifier design eliminates the cost of additional stages of amplification, which could be up to $30,000 to $40,000 per stage, including optics, crystal (gain medium), mount, vacuum parts, optical table, etc. If a second-stage amplifier requires one or more additional pump lasers, the savings is even greater. The cost of a typical pump laser (Positive Light Evolution-30) is about $90,000/20 mJ and approx. $10,000/year for maintenance.
However, present-day laser systems employing single-stage amplification are generally limited to “medium” power output, typically on the order of 1–2 mJ. The potential for damage to the gain medium limits the maximum power output of a typical single-stage amplifier. In order to achieve high gain in a single stage, a very tight focus is employed to maintain a very small effective beam diameter through the gain medium. The signal (pulse) beam is be directed through the highest-gain portion of the pumped volume of the medium (the central portion thereof), achieving single-pass gain levels on the order of 10:1. Unfortunately, however, this technique takes advantage of only a small portion of the inverted gain medium, and achieves relatively low conversion efficiency from pump energy to output pulse energy, typically about 15%. Output stability tends to be poor because of the low efficiency. Further, because of the tight focus, energy density within the gain medium is very high, greatly increasing the risk of damage to the gain medium at higher output power levels.
A typical prior-art high-power pulse laser system employs a multi-stage architecture whereby a low power seed pulse (on the order of nanojoules) is passed through a high-gain preamplification stage and boosted to medium power levels, then through a power amplification stage to produce high output power.
By way of example, a 3-stage system is described by Y. Jiang et al. (Y. Jiang et al, “High-average-power 2-kHz laser for generation of ultrafast x-ray pulses”, Optical Letters, 27, 963 (2002)) produces 1–3 uJ output at 2 kHz with a 1.5 W pump, 2–2.5 mJ output with an 11 W pump; 3–14 W output with a 54 W pump. The system employs two pre-amplifier stages followed by a single power amplification stage. The efficiencies of the three stages are 0.4%, 4.5% and 26%, respectively. Risk of damage to the gain medium is reduced because the amplification is not all achieved in the same stage (The small-signal single pass gain in the first stage is approximately 7:1). The output stability is good because of high efficiency and gain saturation in the final power amplification stage.